All subjects will undergo total knee replacement using same implant (Zimmer NexGen LPS Flex) by same surgeons. All operations will be done under spinal anesthesia injecting 1.8-2.5ml 0.5% Bupivacaine into L3/4 or L4/5 disc space without fentanyl or other analgesics. Pre-operative, perioperative and postoperative analgesic regimen will be standardised.